This invention relates to an adapter for connecting a threaded metal element to a plastic pipe in high pressure systems and particularly to an adapter for connecting a metal sprinkler head to plastic pipe in fire extinguishing sprinkler systems.
Conventional fire extinguishing systems generally comprise a network of piping for delivery of a fire extinguishant such as water to various locations throughout a building through sprinkler heads provided at the end of each pipe segment. The sprinkler heads are activated when the surrounding temperatures exceed an upper limit which causes a low melting point alloy in the sprinkler heads to melt.
Fire extinguishing systems utilizing plastic pipe are usually wet systems because water under high pressure (e.g., 200 psi or more) is maintained in the pipe to prevent heat damage thereto. However, prior pipe systems were frequently subject to failure at the threaded connection between a plastic adapter which is adhesively secured at one end thereof to the plastic pipe and threadably connected to the metal sprinkler head at the other end. The plastic threads of the adapter were found to be subject to attack by the solvent adhesive used to secure the adapter to the end of the plastic pipe. Moreover, cracks in the plastic coupling were frequently encountered due to overtightening of the metal sprinkler head. Cross-threading was also a problem when installing the sprinkler head to the plastic adapter.
To avoid some of these problems, brass inserts with threads have been incorporated into the end of plastic adapters during the formation thereof. Additionally, metal collars have been mounted around the exterior of the plastic adapter in order to provide support and to minimize the cracking thereof when mounting the sprinkler head. While these modifications reduced somewhat the frequency of failures, they were very expensive and very inconvenient to employ in the manufacturing procedures, particularly with conventional injection molding procedures utilized to manufacture the plastic adapters.
What has been needed and heretofore unavailable is a simple, inexpensive adapter to connect plastic pipe to a threaded metal element, such as a sprinkler head, which maintains a sound, leak-proof connection under high pressure systems. The present invention satisfies that need.